Wood-based panel products are used in a wide-variety of applications throughout the world. These applications include commodity structural sheathing for wood construction (e.g., roofs, walls, sub-floors), value-added engineered structural panels/structural composite lumber (e.g., oriented strand lumber (OSL), laminated strand lumber (LSL), parallel strand lumber (PSL), rimboards, webstock for I-Joists, stair stringers, and stair tread), and non-structural core stocks (e.g., decorative panels, furniture, doors and door parts). There are two main types of structural sheathing panels used in structural applications: oriented strand board (OSB) and plywood. The main types of panels used on non-structural applications include particle board, medium density fiberboard (MDF), and high density fiberboard (HDF).
All of these engineered wood products are manufactured with “thermosetting” resins that require cure time in a hot press while under pressure. For this reason, they are made almost exclusively with resins that will not stick to the hot steel of the press, particularly in the surface layers that are directly exposed to the hot press (typically referred to as “platens”). There are a few resins that are commonly used in these applications because they do not stick to the press platens. These resins include: liquid phenol-formaldehyde (LPF), powder phenol-formaldehyde (PPF), urea-formaldehyde (UF), melamine-urea-formaldehyde (MUF), and melamine-urea-phenol-formaldehyde (MUPF).
Another wood binder called MDI (methylene diphenyl diisocyanate or diisocyanate-diphenylmethane) is sometimes used in the manufacture of these products because of its superior binding properties (e.g., can cure under higher moisture conditions, cures relatively quickly, cures at lower temperatures, and has a relatively light color). However, it has an incredible propensity to stick to hot metal. This is especially problematic with the top surface of a panel of a hot press (platen) for each press opening in a multi-opening press. In contrast, the bottom portion of each press opening is not as problematic, because the bottom portion of each press opening of the multi-opening hot press includes a screen, which is flexible and moves over rollers as the panel is moved off the screen.
The propensity of MDI to stick to hot metal typically results in spraying a release agent on the uncured mats before the mats enter the press. This method carries a significant potential risk because if any spot is missed with the release agent spray, would have the risk of sticking to the press and the line must be shut down to remove the panel (e.g., to grind the portion of the panel stuck to the platen off).
Unfortunately, even when they work, there are side-effects with the use of release agents (e.g., soap-based, silicone-based, sugar-based or lipid-based release agents). The silicone-based release agents build up the press surface and eventually interfere with panel pressing in terms of thickness variation and thermal transfer. To avoid this, “soap” based release agents (e.g., potassium oleate) are used, as they are completely sacrificed during the hot pressing. The issue with the soap-based release agents is that they leave a dark discoloration on the surface of the panel that some customers find unsightly. In heavier applications of soap release, excessive smoke is released at the press, and the panels pick up a slight odor that can be offensive.